1. Field of the Invention
This invention relates to rare earth element doped silica glass and more particularly it relates to rare earth element doped silica glass to be suitably used for active optical devices as well as a method for manufacturing the same.
2. Prior Art
Known papers that deal with so-called functional optical fibers having a core of a rare earth element include the following.
Paper No. 1: C. J. Koester and E. Snitzer, Appl. Opt., 3,1182 (1964). PA0 Paper No. 2: S. B. Poole et al., Electron. Lett. 21, p. 738 (1985) PA0 Paper No. 3: R. J. Mears et al., Electron. Lett. 23, p. 10,26 (1987) PA0 Paper No. 4: E. Desurvire et al, Opt. Lett. 12.888 (1987) PA0 Paper No. 5: K. Arai et al., J. Appl. Phys., 59.3430 (1988) PA0 Paper No. 6: B. J. Ainslie et al., Mater. Lett. 6,139 (1988) PA0 (1) U.S. Pat. No. 3,895,073: P. C. Schultz (1975) PA0 (2) U.S. Pat. No. 4,110,093: P. B. Macedo (1975) PA0 (3) U.S. Pat. No. 4,110,096: P. B. Macedo (1978)
The first two papers address fiber laser utilizing light amplification achieved by stimulated emission of excited rare earth elements, while the rest of the papers concentrate on light amplifiers to be prepared by using the technology of fiber laser.
It has been found that optical fibers doped with erbium, a rare earth element, provide a particularly advantageous material for optical amplifiers because optical amplifiers using such optical fibers do not require light-electricity (O/E, E/O) conversion as they can effectively amplify the intensity of light around the wavelength of 1.55 .mu.m currently used for optical communication systems.
However, rare earth element doped functional glass fibers using SiO.sub.2 glass or GeO.sub.2 -SiO.sub.2 glass as a host glass are accompanied by certain disadvantages, which will be described below.
Firstly, rare earth elements cannot be added to glass of this type too much to attain a high concentration level since highly concentrated ions of a rare earth element tends to extinguish the light emission of their own.
This phenomenon, so-called concentration quenching, is due to non-radiative decay process caused by clustered ions of the rare earth element in the glass to reduce the life time and the efficiency of the emission.
Secondly, optical amplifiers using rare earth element doped, or particularly erbium doped, optical fibers have a very narrow light emission spectrum and hence operate satisfactorily only for a limited wavelength bandwidth.
In an attempt to avoid this problem, there has been proposed a method of co-doping glass with both aluminum and a rare earth element in a paper shown below.
The technique disclosed in this paper can provide glass doped with a rare earth element to a relatively high ion concentration without causing the phenomenon of clustering.
Glass co-doped with both a rare earth element and aluminum to achieve high ion concentration of a rare earth element offers the following advantages.
Firstly, it brings forth a sufficient amplification gain if the distance allowed for interaction of pump light and ions of a rare earth element is short. This leads to realization of a compact laser or optical amplifier.
Secondly, changes can occur in the light emission spectrum of ions of a rare earth element by co-doping of aluminum and the element.
More specifically, the light emission spectrum of erbium doped silica glass can be broadened by aluminum co-doping for a wavelength band around 1.55 .mu.m so that an optical amplifier that accommodates a broad wavelength bandwidth can be realized.
This provides a particularly favorable advantage when rare earth element doped silica glass is used for optical amplifiers in wavelength-division-multiplex transmission systems.
Currently available methods for preparing glass preforms to be used for optical fibers which are co-doped with both a rare earth element and aluminum include so-called MCVD solution impregnation method that has been developed from MCVD method and disclosed in a paper shown below.
According to this paper, glass having a lower refractive index is deposited on the inner peripheral surface of a silica glass tube by means of an ordinary MCVD method to form a clad glass layer there and then porous glass is deposited on the inner surface of the clad glass layer to form a core glass layer by means of MCVD method conducted at relatively low temperature. Thereafter, a rare earth element and aluminum in solution are introduced into the pores of the core-forming porous glass layer until they are saturated with the solution.
The solution-impregnated and core-forming porous glass layer is then dried, dehydrated and sintered in a helium gas flow to make it non-porous (transparent vitrification). Thereafter, the obtained silica glass tube having a clad glass layer and a core glass layer is collapsed by known technique to produce a solid rod-shaped optical fiber preform.
Reportedly, a rare earth element can be added to silica glass by more than 3 wt % without clustering ions of the element in the glass.
The MCVD solution impregnation method as described above provides an advantage that oxygen-hydrogen flames can be used to effectively heat the silica glass tube (substrate) that plays the role of a reactor tube to a temperature where the high melting point crystal phase in the doped glass layer disappears, that no cracks are produced by thermal stress if the doped glass is crystallized in the sintering stage because the crystallized glass can be subjected to a collapsing process at or above 1,900.degree. C. without a cooling step and that the crystal phase in the doped glass is completely wiped out during the collapsing process and transparent glass preforms of optical fibers can be obtained by rapidly cooling the glass immediately after the collapsing process.
On the other hand, the above described MCVD solution impregnation method is accompanied by a drawback that homogeneous and optically excellent glass preforms cannot be formed to large dimensions because clad and core glass layers are deposited on the limited inner space of a tube.
The technique of solution impregnation has been known for long for doping and popularly used in recent years for doping using rare earth elements and transition metals that can hardly be added to glass by means of an ordinary vapor phase method.
Improved solution impregnation methods are disclosed in the following documents (1), (2) and (3).
Unlike MCVD method, on the other hand, a so-called outside process involving VAD method, OVD method, sol-gel method, powder molding method or slip cast method has an advantage that it can produce homogeneous and optically excellent glass preforms having large dimensions because it is not by any means restricted by the size of the substrate tube of the preform.
Thus, an outside process may be suitably used to produce rare earth element doped glass laser rods that have a high output capacity and are free from restrictions concerning the shape and size of the glass rod and therefore applied to the manufacture of functional optical waveguides.
For these reasons, porous and vitreous preforms to which dopants are added by means of a solution impregnation method and which are prepared by means of an outside process may offer a wide spectrum of applications.
The inventors of the present invention have conducted a number of experiments to find out if a VAD solution impregnation method is feasible for aluminum doping just as an MCVD solution impregnation method is. Some of the results of the experiments will be described below.